However... the beginning of the year 10 C.E. marked the end of the 9th year since the end of the 1st year B.C.E.
(Say that aloud). The First decade in the Common Era started 2009 years ago, at the beginning of the year 1 C.E.

In Western culture, when you're born, you're 0 years old ( We don't include the first 9 months because most people don't record the day their child was conceived)

In that sense, the end of your 9th year or beginning of your 10th year marks being alive for one decade.

This is cardinal counting and is how modern mathematics counts.
0.....1.....2......3....

However, some cultures (Korean is one) use the ordinal counting system to count years.

1st......2nd......3rd.....4th.....

This is why we live in the 2000s but in the 21st century. It's all because of that pesky 0 that the hindus invented.

I guess I should be thankful... it would've sucked to have to add MDCCLXVI + MDCCCLXXXVII in school.

Google to the rescue:

MDCCLXVI + MDCCCLXXXVII = MMMDCLIII

(highlight the line above to read it - I don't want to spoil the surprise for those of you who are still converting to Arabic, doing your sums, then converting back to Roman)

I was taught that the number 'zero' and the concept of 'zero' wasn't incorporated into western culture until the 1750s, so before then it was '2 BC ... 1 BC ... 1 AD ... 2 AD ...' - there was no concept of 'year zero'. The year the event happened was 1 AD, and the year before was 1 BC, so the first century started 1 AD and ended 100 years later when the second century started in 101 AD; the first Millennium also started in 1 AD, and the second Millenium started 1000 years later in 1001 AD, and the third Millenium started 1000 years later in 2001 AD.

It doesn't help that the popes kept fiddling with the Christian calendar over the centuries, or that the 'definitive' setting of the Gregorian calendar is off by seven years because of mis-calculations made by the Roman Abbot Dionysus Exegines in the 6th Century. Pope Gregory had a hard enough time getting everyone to agree to skip 14 days to fix the calendar (the Eastern Orthodox Church doesn't use the Gregorian calendar - their High Holy days are two weeks after the Western Christians' High Holy days ). Even papal authority couldn't make the calendar skip 7 years too.